Respective to the Print Release portion of the present invention:
The present invention can be applied to many applications, two of which are the application of Solder bumps to Integrated Circuit (IC) Wafers or die and the application of solder paste onto Printed Circuit Boards for populating Printed Circuit Assemblies.
The current process for applying solder bumps to IC wafers or die is extremely inefficient resulting in high assembly costs. The current process involves applying a solder mask to the wafer, plating solder to the wafer, reflowing the plated solder, then removing the solder mask.
As technology advances, components are getting smaller, the size of the solder connections are respectively being reduced, and therefore, the assembly process of Printed Circuit Cards is requiring the application of smaller and smaller deposits of solder paste.
What is desired, as an overview, is a low cost and repeatable process for applying solder bumps directly to an IC as a means to reduce the assembly costs.
What is further desired would be the application of similar technology for the use in populating high tech, fine pitch printed circuit assemblies (PCA's).
A stencil (also referred to as a screen) is created with an aperture or a plurality of apertures defining a pattern that is to be ‘printed’ onto a surface. The stencil is placed onto a surface upon which a material is to be deposited in a pattern. The material may be in a liquid, a solid or a solid/liquid composition. In the case of the preferred embodiment, the material that is to be deposited comprises fine particles of solder powder mixed into flux, commonly referred to as solder paste.
The stencil is generally placed substantially parallel to the surface, and may contact the surface, with the stencil and the aperture(s) aligned to a pattern on the surface to be printed to apply the desired pattern of material. For gravity-driven printing, the stencil is generally placed on top of the surface. The material to be deposited is then placed on top of the stencil for deposition into the aperture(s). Various methods may be used to move the material across the non-contact side of the stencil, placing the material into the aperture(s), as generally known in the art. For example, squeegees are often used in various ways to move material into the aperture or apertures. Once the apertures are filled with material, excess material may be removed from the non-contact side of the stencil so that substantially all of the material that remains is within the aperture(s).
Lastly, the stencil and the surface to be printed upon are separated, transferring the material in the desired pattern onto the surface of the object.
Various methods and equipment have been invented to automate the process described above, with many different approaches, as are known in the art. Many improvements in the art have resulted in an increase in the efficiency of the process. For example, machines have been invented to hold the stencil, align the stencil to the surface to be printed upon, deposit material into the apertures while removing any excess material, and finally separate the stencil and the surface being printed upon. However, none of these has solved the problems that the present invention solves to ensure a repeatable amount of material is transferred to the surface being printed upon.
The screen printing process is made more difficult as the size of the apertures decreases and the size of the surface area on the objects decreases. This is particularly true in manufacturing printed circuit assemblies and applying solder bumps to IC wafers or die. One particular problem that the present invention solves is that the material to be deposited onto the surface tends to stick to the sides of the apertures in the stencil. This problem has several particular outcomes that the present invention solves. First, the deposited material may slump or otherwise move outside the area defined by the aperture after the stencil is removed. For electronic assemblies, this can have disastrous consequences and require rework of defects. Secondly, the shape of the remaining material may cause problems. Preferably, the material that is left on the surface will have a uniform surface defined by the outside of the stencil, and the remaining top surface of the material will be substantially flat. Third, it is desired that the maximum amount of material be transferred of the material placed into the apertures to the object. For PCBs, the surface on which the material is to be deposited may generally be referred to as a pad. Uniformity is very important for pads so as not to create an area of conductivity where that is not desired. Further, it is important that the pad have a uniform top surface to enhance the attachment of electronic components. Forth, the apertures must remain void of any excess material to ensure repeatable transfer of material is accomplished.
FIGS. 1A, 1B, 1C and 2 are simple illustrations that show some of the basic steps in the process. The geometrical problem from a FIG. 1A shows a three-dimensional view of a stencil. FIGS. 1B, 1C, 1D and 2 show two-dimensional views. FIG. 1A shows a section of a stencil 20 that has aperture(s) 22 registered to pad(s) 14 on a Printed Circuit Board 10. FIG. 1A further illustrates the material deposition surface 12 of the Printed Circuit Board 10 as well as some circuitry 16. FIG. 1B shows a single aperture 22 in the section of the stencil 20 from a cross sectional view, during a registration step, registered, but some distance to the material deposition surface 12. The aperture(s) 22 comprises an aperture side-wall 24. FIG. 1C shows the same section of the stencil 20 placed against the material deposition surface 12 and a material 26 has been placed in the aperture(s) 22. Finally, FIG. 1D shows the stencil 20 that has been lifted off the surface 12, leaving behind the material 26. The surface area of the aperture side-wall 24 of the aperture(s) 22 in each of FIGS. 1A, 1B, 1C and 1D are relatively small compared to the target area 18 of the material deposition surface 12 defined by the aperture(s) 22. When the stencil 20 is removed after material 26 is placed in the apertures, gravity and surface effects cause most of the material 26 to stick to the target area 18. To a lesser extent, surface effects cause the material 26 to stick to the aperture side-wall 24. If the target area 18 is much larger than the sides of the aperture(s) 22, the effect of material sticking to the aperture side-wall 24 is of less practical concern.
However, FIG. 2 illustrates the problem presented when one uses a thicker stencil 30—the surface area of a taller aperture side-wall 32 of the aperture(s) 22 become relatively larger when compared to the target area 18 of the material deposition surface 12. This is generally due to shrinkage in the size of the components to be mounted or the density of the leads of the components. Here, the surface tension effects of the material 26 contacting the taller aperture side-wall 32 are relatively larger, resulting in a tendency for the material 26 to stick to the taller aperture side-wall 32 of the aperture(s) 22, causing a number of problems or potential problems. As mentioned earlier, the material may slump and migrate outside the area defined by the aperture causing conductivity problems. Further, the surface area of the resulting deposit may not be uniform (illustrated later), potentially creating problems in attaching components. And lastly, the maximum transfer of the material from the aperture(s) 22 to the material deposition surface 12.
The limitations of this process continue to be challenged as the aperture(s) 22 and the resulting target area 18 (generally an area respective to the pad(s) 14) decrease in size. There are factors other than geometry that may impact the release of the material. Examples include the shear to tact ratio of the material, the surface finish of the stencil, cleanliness of the stencil, and the cross sectional geometry of the stencil aperture.
To date, attempts to solve the problem have focused on changing the stencil release speed, changing the surface finish of the stencil, and changing the cross-sectional geometry of the aperture(s) 22 using a aperture side-wall 24 that is non-vertical.
A first known method is the use of a slow separation between the thicker stencil 30 and the material deposition surface 12. The slow separation utilizes gravity to assist in the release of the material 26 by allowing the weight of the material 26 to overcome the shear force (illustrated in a later figure) at the interface between the material 26 and the taller aperture side-wall 32 of the aperture(s) 22 of the thicker stencil 30. The detriment of this approach is that it inherently increases the cycle time of the process.
A second method known to assist with release of the material 26 is to modify the surface of the aperture side-wall 24, 32 of the stencil 20, 30. Two examples of this applied to metal stencils would be electropolishing and nickel-plating the surface after creating the aperture(s) 22.
A third known method to assist with release of the material 26 is to design the cross section of the aperture(s) 22 in a trapezoidal shape, where the area defined by the aperture(s) 22 at the PCB contact side of the stencil is larger than the area defined by the aperture(s) 22 at the solder side of the stencil.
Thus, what is desirable, is a means to increase the speed of release of the material 26 without altering the geometry of the deposition, ensuring maximum transfer of the material 26 from the aperture(s) 22 to the object, and increasing the quality of the resulting material 26 deposition.
Respective to the Cleaning Portion of the Present Invention:
Fluid based cleaning systems are commonly used for cleaning Printed Circuit assemblies (PCA's), Wafers, and PCA Assembly Tooling. Hot air drying systems are an established method of drying bare Printed Circuit Assembly (PCA's), various components on a (PCA), and tooling which may require cleaning such as stencils, board supports, and the like. There are a wide variety of equipment and processes available to manufacture, solder, clean and dry PCA's, however, the general principles of the process remain the same, as explained below.
Components may be surface mounted to the PCA utilizing solder paste which may contain flux used to deoxidize the surface mount pads on the PCA and the leads of the components. The powder of solder is fused during a reflow process, creating the electro-mechanical connections. During this process, the flux is activated, where some of the ingredients of the flux evaporate, leaving a residue on the assembly, referred to as a module. Components may additionally be assembled to the PCB using a wave solder process, where leaded component are assembled by placing the leads into non-plated and/or plated through holes and/or surface mount components are glued to a bottom surface of the PCA. This assembly is then wave soldered to the PCA utilizing flux to deoxidize the leads and plated through holes. Flux residue and other contaminants may remain on the module. The module may then be cleaned in an aqueous cleaning system. Cleaning may be used to remove flux residue or other contaminates such as solder balls associated with the component or module manufacturing process. Once cleaned, it is important to remove all of the moisture from the interior of open components and the exterior of the module.
The limitations of this process continue to be challenged with the inclusion of smaller openings within connectors, smaller gaps under components, and the like which can entrap moisture. Any excess water or moisture will cause corrosion over time. This is especially a problem when power is applied to a module which is not dry, causing a galvanic reaction and, therefore, corrosion.
Tooling, such as solder stencils and wave solder pallets, require cleaning as become contaminated with either solder paste or flux residue. Solder stencils need to have any remaining solder paste removed prior to storage. If the solder paste dries within the apertures of the stencil, the dried solder paste will interfere with the release of the solder paste during the next assembly process and cause defects. Solder stencils are not currently used for applying solder paste or solder spheres to IC Wafers, as the cleaning process is very time consuming as well as incomplete. Build up of flux residue on wave solder pallets will hinder the application of the flux onto the assembly and cause defects.
The solder stencil printing process sometimes includes an under wiping process. The under wiping process may further apply a solvent to either the under wiping paper or the underside of the stencil. The under wiping process can further include a vacuum system which removes the loose solder particles located inside the apertures of the stencil and any solvent in the direct flow of the vacuum. The under wiping process may not sufficiently remove excess solvent remaining on the top-side of the stencil. The process may not remove the solder residue within the apertures of the stencil.
After completion of the reflow and/or wave soldering processes, the assemblies are cleaned to remove the remaining residue or contaminants. The cleaning process applies some form of liquid, generally de-ionized water. Chemicals with relatively low flash points were used in the past, but those chemicals are expensive and some were found to be harmful to the environment. One of the more desirable chemicals used to clean assemblies is water. Water, or other cleaning solutions with similar flash points, is difficult to dry in a short time period. The desirable outcome of the drying process is for components and the module to be sufficiently dried to preclude corrosion. Various processes and devices are available to dry electronic modules.
In one case, hot air is blown over and across the module with sufficient velocity, volume and thermal content to evaporate some of the moisture and urge some of the remaining moisture off the module. The limitations of this are that the dryers require a great deal of thermal energy and large capacity air blowers to provide sufficient drying. Additionally, these dryers are generally loud and require sound dampening. Drying depends on convection of hot gases past the module. The rate of drying decreases after a portion of moisture has been removed. The last few points of moisture removal take the longest and increase the cost of drying. If one attempts to increase the temperature of the drying gas, there is a risk of thermally damaging the electronic components on the module. The efficiency of drying is proportional to the temperature of the drying gas. Thermally damaging the module sets a practical upper limit for the gas temperature. Additionally, this process continues to be limited when moisture is trapped in components such as connectors.
In another case, infrared energy is applied to the module in an attempt to evaporate excess moisture. This process is somewhat limited by the time required to dry any excess moisture. Because of this limitation, infrared dryers are often used in conjunction with hot air dryers. Infrared energy transfers heat to the exposed surfaces; where the infrared energy would have a difficult time to evaporate entrapped moisture from within pockets of components such as connectors or under components designed to have a space between them and the surface of the PCB, such as ball grid array packages and Direct Chip Attach or Flip Chip.
Solder Stencil printers do not have any known means of drying other than the paper used to wipe the underside of the stencil, and the under wipe paper is not conducive to removing moisture.
In another case, reference is made to U.S. Pat. No. 5,228,614 which teaches a method of drying objects in a perforated drum. Hot gas and sonic energy are used to dry the food objects which are tumbled in the perforated drum, and upon sufficient drying, the objects are removed from the drum. The limitations of this patent are that electronic modules cannot be tumbled in a drum and are most often processed on a conveyor to preclude damage to the module.
In yet another case, reference is made to U.S. Pat. No. 3,592,395 filed Sep. 16, 1968, to Lockwood, et. al. This dryer uses pulsating hot gas and sonic energy to dry a stirred slurry. This dryer readily handles slurries or other fine powdery materials. This type of dryer would not work with electronic modules as any stirring of electronic modules would cause mechanical damage to the modules.
In yet another case, reference is made to U.S. Pat. No. 5,113,882 filed Aug. 28, 1990 to Gileta. A dryer system for a liquid cleaning apparatus has a dehumidifier to remove vapors, droplets of liquid cleaning agent and recirculate dry gas onto workpieces moving on a conveyor. Gileta teaches lowering the relative humidity within the atmosphere to increase the efficiency of the drying of printed circuit assemblies. Ultrasonic transducers are used in wave soldering technology to atomize liquid flux into a fine mist and transferring the flux in mist form from the source reservoir to the bottom side and into the plated through holes of the module. This is commonly referred to as a spray fluxer.
It can be recognized that improvements made to the drying process of modules, can also be utilized in the drying processes applied to tooling as well as stencils within solder printers.
While each of these improvements has contributed to the art, the limitations of these processes continue to be challenged.
Thus, what is desirable, is a means to reliably clean and dry electronic modules and tooling utilizing a minimal amount of energy and time and precluding any mechanical or thermal damage to the module.